A growing literature documents the importance of student teaching placements for teacher development. Emerging evidence from this literature highlights the importance of the mentor teacher who supervises this placement, as teachers tend to be more effective when they student teach with a mentor who is a more effective teacher. But the efficacy of policies that aim to have effective teachers serve as mentors depends a great deal on the availability of effective teachers to serve in this role. We therefore use data from Washington State to illustrate that there is ample scope for change in student teacher placements; in other words, there are far more effective teachers within 50 miles of a teacher education program (TEP) who could host a student teacher in each year than the number of teachers who serve in this role. We also discuss the considerable challenges to improvement efforts related to the need for better coordination between TEPs, K-12 school systems, and states. Finally, we argue that, if policymakers value teacher "candidate" development equivalently to teacher "in-service" development, they should be willing to pay substantially more than the current average compensation for mentor teachers to recruit effective teachers to serve in this role. [This article was published in "Education Finance and Policy" (EJ1259717).]